Microporous membranes are usually defined as thin walled structures having an open spongy morphology with a narrow pore size distribution. The mean pore sizes for microporous membranes typically range between 0.01 .mu.m and 10 .mu.m. Traditionally, microporous membranes are used to remove fine particulate matter such as dust and bacteria from liquids and gases. A microporous membrane can achieve the clarification by different mechanisms. For example, particulates can be retained by microporous membranes through physical sieving of all particulates larger than the pore size of the membranes. In this mechanism, filtration efficiency is governed by the relative size of the particulates and membrane pore size. Due to the increasing need for removing finer and finer particulates, particularly in pharmaceutical and electronic industries, microporous membranes with very small pore size are required to achieve effective filtration. However, membranes with such a small pore size tend to have some undesirable characteristics such as high pressure drop across tee membrane, decreased flow rate, reduced particulate capture capacity, and shortened membrane life.
Another mechanism in which a microporous membrane can remove suspended particulate materials is the electrokinetic capture mechanism. This is achieved by imparting an appropriate zeta potential to the membranes' internal and external surfaces. In principle, when a charged surface is immersed in an aqueous medium or other polar medium, a charge double layer will form at the solid-liquid interface. One component of the double layer is the charged solid surface, and the other layer is the counter ionic region in the medium. When the solid and liquid are set in relative motion, a potential difference will develop between the mobile and immobile regions in the medium close to the surface. This potential, the so-called "zeta potential" is given for a fluid flowing through a charged porous membrane by the equation: ##EQU1## where .zeta. is the zeta potential, .eta. is the solution viscosity, D is dielectric constant, E is the streaming potential, P is the driving pressure and K is the specific conductance of the solution.
The zeta potential can be either positive or negative depending on the charge of the solid surface. Most suspended particulates which are commonly subjected to removal by filtration have a negative zeta potential. Therefore, such particulates will be readily adsorbed or attracted by solid surfaces that have positive zeta potentials. Based on this, applying a positive zeta potential to the available surface of a microporous membrane will greatly improve the particulate capture capacity of the membrane. This is true even for particulates whose size is much smaller than the membrane pore size. As a result, a high fluid flow rate through the membrane can be maintained using this concept and yet particulate capture by the membrane is much more efficient than indicated by the rated pore size of the membrane.
Attempts to enhance flow rates and to increase membrane life using cationically charged membranes have been made for a number of years. For example, the U.S. Pat. Nos. 4,007,113, 4,473,474, 4,673,504, and 4,708,803 to Ostreicher et al. describe the use of a charge modified filter and process for making the same. U.S. Pat. No. 4,473,475 to Barnes et al. also disclose a cationically charged microporous membrane and its usage. U.S. Pat. No. 4,523,995 to Pall et al. and U.S. Pat. No. 4,604,208 to Chu et al. are other examples of charge modified microporous membranes.
Each of the above mentioned patents is limited to the use of charge modified membranes in filtration applications. However, such charge modified microporous membranes can be used for macromolecular transfer application (e.g., DNA Southern blot) and have already been suggested in U.S. Pat. Nos. 4,512,896 and 4,601,828 to Gershoni and in European patent application 0347755 to Pluskal et al.
The term "macromolecular transfer" as used herein refers to processes for transferring biological macromolecules such as nucleic acids and proteins from electrophoresis gels to some type of immobilizing matrix. Of particular importance is nucleic acid blotting, such as DNA blotting. A variety of DNA blotting techniques have been developed in the past. Among them, the most common is referred to as "Southern blotting" in which DNA fragments are separated by chromatographic techniques and then denatured while still in the gel. The gel is neutralized and placed over wicking papers which are in contact with buffer held in a buffer reservoir. The blotting membrane is then placed on top of the gel. As the buffer flows into the gel, DNA is eluted and binds to the blotting membrane, thereby transferring the DNA fragment pattern onto the blotting membrane. The fragment pattern can finally be detected using hybridization techniques employing labeled nucleic acids which are complementary to the specific bound fragments.
DNA blotting membranes presently available are limited to nitrocellulose, charged nylon, charged polyvinylidine difluoride, and activated papers derivatized with diazo containing compounds.